The present invention relates to acicular magnetic metal particles containing iron as the main ingredient which are used for high-density recording and which are excellent in switching field distribution (hereinunder referred to as "S.F.D.") and dispersibility, and are capable of producing a high output, and a process for producing such particles.
The present invention also relates to acicular magnetic metal particles containing iron as the main ingredient which have a high coercive force, a high saturation magnetization, a fine particle size and excellent S.F.D., and a process for producing such particles.
With the recent development of smaller-sized and lighter-weight magnetic recording apparatuses, the necessity for recording media having a higher performance such as a magnetic tape and a magnetic disk has been increasing more and more. In other words, magnetic recording media is required to have a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lowered noise level. In order to satisfy the above-described demands on magnetic recording media, magnetic particles used in the production of the magnetic recording media must have a high coercive force, a high saturation magnetization, a fine particle size and an excellent S.F.D.
To meet such demands, development of magnetic particles which are suitable for high-output and high-density recording, namely, magnetic particles having a high coercive force and a high saturation magnetization, has been pursued. As magnetic particles having the above-described properties, acicular magnetic metal particles containing iron as the main ingredient obtained by heat-treating as a starting material acicular ferric oxide hydroxide particles which are obtained by oxidizing a neutralized precipitate of a ferrous salt and an aqueous alkaline solution such as alkali hydroxide and alkali carbonate, or acicular hematite particles which are obtained by heat-treating the thus-obtained acicular ferric oxide hydroxide particles in a reducing gas so as to produce acicular magnetic metal particles containing iron as the main ingredient; and forming an oxide layer on the surfaces of the particles, are known and put to practical use.
The coercive force of acicular magnetic metal particles is dependent on the shapes of the particles, in particular, on an aspect ratio (major axial diameter: minor axial diameter) and has a tendency of increasing with the increase in the aspect ratio. The saturation magnetization has a tendency of increasing with the acceleration of reduction, for example, by raising the reducing temperature. The coercive force and the saturation magnetization have reverse correlation. That is, while the saturation magnetization is enhanced with the progress of reduction, since the heat-treatment is carried out under the severe conditions, the particles grow and sintering is caused on particles and between particles, and as a result the shapes of the particles are lost, thereby reducing the coercive force. Thus, acicular magnetic metal particles having both high coercive force and large saturation magnetization are strongly demanded. Acicular magnetic iron particles are also required to have excellent dispersibility, and it is therefore necessary that sintering of particles and between particles are prevented during the heat-treating step and the oxide layer producing step after heat-treatment so as to maintain the shapes of the particles, and that the particles are independently distributed.
Although acicular magnetic metal particles have a high coercive force and a large saturation magnetization, they are very unstable. More specifically, since the acicular magnetic metal particles used for magnetic recording media are very fine particles having a particle size of not more than 1 .mu.m, the surface activity of the particles is so large and when such acicular magnetic metal particles are taken out into air after reduction, they rapidly react with the oxygen in air, thereby igniting. In addition, by such oxidation reaction these particles are converted into oxides, thereby greatly reducing the magnetic characteristics, in particular, the saturation magnetization, so that it is impossible to obtain acicular magnetic metal particles having a large saturation magnetization aimed at.
It is widely known that the noise level of magnetic recording media has a close relation with the particle size of the acicular magnetic metal particles used in the production of the magnetic recording media and the crystallite size, and that the noise level has a tendency of lowering in proportion to the reduction in the particle size and the crystallite size.
Furthermore, in order to increase the output of magnetic recording media, in addition to the improvement of the magnetic characteristics, the acicular magnetic metal particles are required to have excellent S.F.D.
As is seen from the descriptions in Japanese Patent Application Laid-Open (KOKAI) No. 63-26821 (1988), if the relationship between the S.F.D. of magnetic recording media, and the recording and reproducing output is represented as a graph, as the S.F.D. becomes smaller, the recording and reproducing output linearly increases, which this fact means that the recording and reproducing output is enhanced by using magnetic particles having a small S.F.D. In order to obtain a larger output, an S.F.D. of not more than 0.6 is required.
It is the general tendency that the smaller the particle size of the acicular magnetic metal particles containing iron as the main ingredient is, the coercive force generally tends to increase and the noise level lowers. Bu since the surface activity of the particles becomes greatly large, when forming an oxide layer by an ordinary method, the reaction with the oxygen in air is rapidly carried out, and as a result the ratio of the oxide layer to the particles relatively increases and, in addition, the oxide layer becomes coarse and non-uniform. Accordingly, the magnetic characteristics, in particular, the saturation magnetization reduced to a great extent, and the coercive force distributes, thereby lowering the S.F.D. This phenomenon is apt to be caused as the particle size becomes smaller.
Various methods of improving the properties of acicular magnetic metal particles have conventionally been adopted. A method of preventing the sintering of the particles and between the particles in the heat-treatment step, comprising coating the surfaces of the particles in advance with various organic compounds or inorganic compounds having a sintering preventive activity is known. In the methods disclosed for example, in Japanese Patent Publication Nos. 60.17802 (1985), 53-11512 (1978), Japanese Patent Application Laid-Open (KOKAI) No. 51-106669 (1976), 52-9900 (1977), 52-30758 (1977), 55-85605 (1980), 55-85606 (1980), 57-60001 (1982), 58-113305 (1983), 59-207843 (1984) and 59-213626 (1984), a silicon compound is used as the compound having a sintering preventive activity.
Also, a method of improving the oxidation stability of the acicular magnetic metal particles containing iron as the main ingredient by contacting the acicular magnetic metal particles containing iron as the main ingredient after reduction with a gaseous organic silicon compound is reported in Japanese Patent Application Laid-Open (KOKAI) No. 60-154502 (1985). In this method, however, since a silane compound having a strong hydrolytic activity is used and the hydrolysis reaction is accelerated in the positive presence of water so as to form an SiO.sub.2 coating layer, the hydrolysis reaction rapidly progresses and it is impossible to obtain acicular magnetic metal particles containing iron as the main ingredient which adequately satisfies the above-described properties.
As a result of the present inventors' studies for achieving the object of providing acicular magnetic metal particles containing iron as the main ingredient which have excellent S.F.D. and dispersibility, it has been found that the acicular magnetic metal particles containing iron as the main ingredient obtained by (1) heat-treating in a reducing gas acicular ferric oxide hydroxide particles or acicular hematite particles obtained by heat-treating the acicular ferric oxide hydroxide particles, thereby obtaining acicular magnetic metal particles containing iron as the main ingredient; (2) contacting the acicular magnetic metal particles containing iron as the main ingredient with at least one of the silicone compounds represented by the following formula (I) in a gas phase: EQU (R.sup.1 HSiO).sub.a (R.sup.2 R.sup.3 SiO).sub.b (R.sup.4 R.sup.5 R.sup.6 SiO.sub.1/2).sub.c (I)
(wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5 and R.sup.6 respectively represent a hydrogen atom or hydrocarbon group having 1 to 10 carbon atoms which may be substituted by at least one halogen atom, a and b are respectively 0 or integers of not less than 1, and c is 0 or 2, provided that when c is 0, the sum of a and b is an integer of not less than 3) so as to coat the surfaces of the particles in advance with a polymer formed from the silicone compounds represented by the formula (I) and treating the acicular magnetic metal particles containing iron as the main ingredient in an oxygen-containing inert atmosphere, thereby forming an oxide layer on the surfaces of the particles, or (2') coating the surfaces of the particles of the acicular magnetic metal particles containing iron as the main ingredient with a polymer formed from the silicone compounds represented by the formula (I) while forming an oxide layer on the surfaces thereof, can achieve the above-described object.
The present invention has been attained on the basis of this finding.